I am reading a Boeing 737 manual and it keeps mentioning different navigation technics and when each is used.
What exactly are the differences between RNAV, VNAV, and LNAV? when is each technique recommended to be used?

RNAV is short for 'Area Navigation'. In the old days you used to have to fly directly over the navigation aids on the ground (VOR, NDB etc) to make your route. It meant a slightly zig-zag course for your flight as you couldn't get the navaids in a perfect line between every possible city pair. Then when computers started getting into planes, it was possible for the computer to 'create' an imaginary navigation aid based on a direction and distance from a real one on the ground. So you could draw a straight line from your origin to destination, and create the waypoints based on the computer figuring the direction and distance from some nearby navaids (usually VORs) and using that to fly a straight route. Nowadays, RNAV is also loosely used to describe any 'straight line' navigation method like GPS, as well as the old RNAV method too.

LNAV and VNAV are parts of the flight guidance system, and are acronyms for 'Lateral Navigation' and 'Vertical Navigation'. I don't know the 737 very well, but I'd bet the 767 I fly is very similar.

LNAV is the route you fly over the ground. The plane may be using VORs, GPS, DME, or any combination of the above. It's all transparent to the pilot, as he enters his route as specified in the clearance and flight plan into the FMS (Flight Management System). The route shows up as a magenta line on the lower flight display, and as long as the autopilot is engaged in the LNAV mode, it will follow that line across the ground. LNAV however does not tell the plane what altitude to fly, and that is where...

VNAV comes in. Vertical Navigation is where the specified altitudes at particular waypoints are entered into the FMS, and the computer figures the best way to accomplish what you want. For instance, if you are flying with the autopilot on in VNAV mode at cruise altitude, you can enter what speed you desire to make a descent at, and what altitude you wish to cross a particular point, and the computer will figure out where to bring the throttles to idle and begin a descent, to allow you to cross to that point in the most economical manner. VNAV also works in climb. There are airspeed restrictions at various altitudes, and if you are in VNAV, it will fly the plane at the desired power setting and angle to achieve the speed (and efficiency) you wish.

In reality, we spend most of our flying with both LNAV and VNAV engaged. If the autopilot is off, LNAV and VNAV still send their signals to the flight director so we can hand fly the plane the way the autopilot would if it were flying.

So in summary, RNAV is a method of navigation, and LNAV/VNAV are subsystems of the autoflight system. LNAV is the course (in 2 dimensions) across the ground, and VNAV is the flight path (in 2 dimensions) up and down. Of course we can do it by other methods which worked well for many, many years. But I've found the computer can almost always do it better and smoother. A word of caution is always given to pilots when first learning the LNAV/VNAV system though; it's best to study well and always keep an eye on what it's doing. It is only as good as the person punching the buttons, and the most common thing heard in today's modern cockpits is "What's it doing now???"

HAL

One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.

the most common thing heard in today's modern cockpits is "What's it doing now???"

I thought that was just in Airbus cockpits?

The 757/767 is very similar to the 737 in this regard (particularly the 737NG) and you can find quite a lot of relevant information on LNAV / VNAV / FLCH and other modes, their interaction with one another and the typical procedures used at http://www.757.org.uk/sops/index.html

Change the 757 to 737 in the website above and you'll have that plane.
The mode names mean the same things but the NG does different things to arrive at the same point. Actually the FMC on the planes is the same hardware and it's the software that makes it do what it does, how it does, and when it does.

http://www.b737.org.uk/
The people that put this website together have done everyone a great service. There's information in there that actually makes sense of the crapppppy AMM's and SDS's thanks to ATA 104 and simplified English. (Couldn't pass up a chance to slam the archetects of "our" new language.)

One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533

In approach if LNAV is selected, will the aircraft carry out all the turns automatically? How come VNAV has to be deselected, at least on the company ops i'm reading is it because encase the autopilot busts minimums?

Quoting Novice,reply=4:How come VNAV has to be deselected, at least on the company ops i'm reading is it because encase the autopilot busts minimums?

There are typically limitations on VNAV. For example it may only be permitted a certain altitude AGL. The margin of error may be too small to have it engaged all the way to the ground for example.

Depends entirely upon that individual company's procedures and/or the limits of the approach being flown. When flying an approach using VNAV, AA 738 normal procedures are to leave VNAV engaged throughout the entire approach. The vast majority of approaches I fly have a VNAV path that takes you to 50 ft above the runway threshold and include a missed approach procedure with VNAV altitudes (if you get that far).

Quoting AAR90 (Reply 7):There are typically limitations on VNAV. For example it may only be permitted a certain altitude AGL. The margin of error may be too small to have it engaged all the way to the ground for example.

Depends entirely upon that individual company's procedures and/or the limits of the approach being flown.

Quite. But it also depends on the autopilot itself. Some can't flare so you really don't want them engaged all the way to the runway.

"There are no stupid questions, but there are a lot of inquisitive idiots."

The flare is the last maneuver performed before touchdown. You're coming down at the reference speed. Just above the runway, you lift the nose in order to decrease descent rate and speed. That's the flare.

"There are no stupid questions, but there are a lot of inquisitive idiots."